1. Field of the Invention
The present invention relates to a biological method for removing uranium from water.
2. Prior Art
Two biological methods have been previously described for removing uranium from water. One method is biosorption. In this technique the water is passed through the biomass of living or dead, usually dead, bacteria, fungi, yeast or algae. The uranium binds to the biomass through passive physical-chemical mechanisms. Microbial enzymatic activity is not directly involved. Patents which describe biosorption processes include U.S. Pat. No. 4,789,481 to Brierly et al and U.S. Pat. No. 4,530,763 to Clyde et al.
A major disadvantage of the biosorption method is that in most waters uranium is in a carbonate complex and uranium in such a complex does not readily adsorb onto biomass. Other disadvantages of biosorption methods include: 1. The limited capacity of biomass for uranium sorption. 2. The fact that other metals and cations may compete with uranium for adsorption sites and limit the extent of uranium adsorption. 3. The fact that biomass is expensive to generate and the method is only cost effective if waste biomass from some other process can be obtained. 4. The biosorbed uranium represents a large volume of waste of which only a fraction is uranium. 5. Although it is possible to regenerate some forms of biomass by extracting the uranium with salt solutions or acids, this extraction is expensive and can result in a large volume of corrosive, uranium containing waste. 6. Biosorption only poorly extracts uranium when it is present at low concentrations.
A second biological method for removing uranium from waters is to add glycerol-2-phosphate to the uranium-containing water and then treat the water with the microorganism Citrobacter sp. Citrobacter sp. has a phosphatase enzyme that releases phosphate from the glycerol-2-phosphate. The phosphate then forms an insoluble uranium precipitate on the cell surface.
Although this process involves an enzymatic reaction, the enzymatic reaction does not involve uranium. Disadvantages of this method are similar to those with biosorption: 1. It is hindered by the presence of carbonate. 2. It precipitates metals, other than uranium, that form an insoluble phosphate complex. 3. The amount of uranium that can be sorbed onto the cell surface is limited. 4. The sorbed uranium represents a large volume of waste of which only a fraction is uranium.
A nonbiological method that has been used for removing low levels of uranium from waters is ion exchange. In this method uranium is adsorbed onto various ion exchange resins. Disadvantages of this method are: 1 The ion exchange material has a limited capacity for adsorption. 2. Other metals and ions are adsorbed which limits the effectiveness and lifetime of the ion exchange material for uranium. 3. Uranium held in complexes may not be extracted with ion exchange materials. 4. Uranium adsorbed onto the ion exchange material produces a large volume of waste that must be disposed of, with only a small portion of the volume actually comprised of the uranium. 5. Removal of the uranium from the ion exchange material with high concentrations of salt or acid produce a highly corrosive uranium-containing waste and the use of such extractants is expensive. 6. Ion exchange resins only poorly extract uranium from water when uranium as in low concentrations. 7. Ion exchange materials are expensive.